It is known that charging/discharging a lithium ion battery involves movement of electrons and lithium ions in the battery and the charge/discharge characteristics of the battery can be enhanced by increasing the moving speed. Since movement of electrons and lithium ions is most disturbed when they move inside an active material particle, the charge/discharge characteristics can therefore be effectively enhanced by reducing the particle diameter of the active material particle to a nanoparticle size so as to decrease the moving distance of electrons and lithium ions in the particle and increasing crystallinity of the active material particle so that electrons and lithium ions can smoothly move in the particle.
The technique for nanoparticulation of the active material particle includes a breakdown-type technique of obtaining a nanoparticle by crushing an active material particle having a particle diameter on the micrometer order, and a buildup-type technique of directly producing an active material nanoparticle from the raw material. The breakdown type is a method of crushing a moderately large active material by using a media dispersion such as ball mill and bead mill and is advantageous in that the method is easily adopted to any materials, but on the other hand, is disadvantageous in that a problem of contamination with media debris is likely to occur in the process of crushing or a long time treatment is required so as to obtain a nanopartilce having a particle diameter of less than 100 nm.
Accordingly, a nanoparticulation technique of buildup type is being studied these days. Many of buildup-type methods are a technique of synthesizing a nanoparticle in a liquid phase and, specifically, a method of dissolving or dispersing the raw material in a solvent and heating the solution or dispersion while stirring to produce an active material nanoparticle.
The method for increasing crystallinity of the active material particle includes a method of heating/annealing the particle, but when a nanoparticle is annealed, it is likely that the particles grow by sintering to each other and the particle diameter becomes large. Accordingly, a method of synthesizing a particle in a pressurized state to obtain a particle with high crystallinity immediately after the synthesis has been proposed.
The method disclosed in Patent Document 1 is a method called hydrothermal synthesis, where the raw material is dissolved in water and the resulting solution is sealed in a pressure-resistant vessel and heated to produce a nanoparticulated active material of a lithium ion secondary battery.
The method disclosed in Non-Patent Document 1 is a synthesis method called supercritical synthesis, where a solution prepared by dissolving the raw material is brought into a high-temperature/high-pressure supercritical state to produce a nanoparticulated active material for a lithium ion battery. In the methods of Patent Document 1 and Non-Patent Document 1, the reaction system during synthesis is exposed to high pressure and therefore, the particle obtained tends to have high crystallinity and to be reduced in the particle diameter.